Guard ring diode



March 10, 1959 PAPP 2,877,374

GUARD RING DIODE FiIed Aug. 2a, 1956 VOLTAGE AMP.

IN V EN TOR. GEORGE PAPP ATTORNEY United States Patent GUARD RING DIODE George Papp, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation Application August 28, 1956, Serial No. 606,776 4 Claims. (Cl. 313-313) This invention relates to electron discharge devices, and is particularly directed to diodes, with specific attention to uniformly logarithmic current-voltage characteristics over a wide operating range.

The electronic art has heretofore been content with poor approximations of the exponential relation between anode voltage and anode current. Space charge effects limit the logarithmic responses at high current densities while leakage currents within the envelope concealv it at low current densities. Non-uniformity of currentvoltage characteristics is further aggravated by the boundary effects at the edges of the emitter and anode. This is due to the random direction of electrons moving between the cathode and anode. Straight parallel electric lines of force should be established between the anode and cathode, but unfortunately the lines are curved at the boundaries of the electrodes. A still further cause of non-uniformity of the field at the boundary between the emitting and non-emitting portions of the heated cathode is due to the difference in contact potential of these portions.

The object of this invention is to provide an improved thermionic diode, the diode being particularly improved in the matter of uniform, predictable, exponential current-voltage characteristics.

The objects of this invention are attained by a cathode with an extended electron-emitting surface and an anode uniformly spaced from said emitting surface but with the anode surface being less in extent than the emitting surface and overlying a portion of the emitting surface well within all of the boundaries of the emitting surface. Auxiliary anodes in the plane of and insulatingly spaced from the edges of the anodes are connected externally with a separate lead-in wire so that the measured anode current can be isolated from the boundary currents to the auxiliary anodes. The diode thus constructed exhibits current-voltage characteristics unspoiled by boundary effects.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

The single figure of the drawing is a vertical halfsection of one electron discharge device constructed according to this invention.

The diode specifically shown in the drawing comprises a tubular envelope 1 with the press 2 at one end and the exhaust tip-off seal 3 at the other end. The electrode mount sealed in the envelope is superficially similar to conventional mounts in that the electrodes are insulatingly supported concentrically in the envelope on spacers 4 and 5. Conventionally, the spacers may be discs of mica. Centrally in the mount is the elongated cathode sleeve 6 containing the helical heater 7.

The sleeve is exteriorally coated with a layer 8 of electron-emissive oxides. This layer may comprise the usual barium-strontium-oxides. Importantly, the oxide layer isapplied uniformly over the sleeve but terminates short of the ends of the sleeve and short of the spacers 4 and 5.

The anode 9 of the diode illustrated is tubular, concentric with the cathode, and terminates short of the ends of the coated area 8. The anode 9 is coaxial with auxiliary tubular anodes 10 and 11, the ends of the anode sections being spaced apart sufiicient for electrical isolation. Metal brackets 12 and 13 mechanically join the three anode sections, the electrical continuity of the brackets being interrupted by glass beads 14 and 15. Other mechanical attachments with electrical insulation could be employed without departing from the scope of this invention. The advantage of the glass beads ex teriorally of the anodes resides in the fact that the insulating surfaces of the beads are effectively shielded from the cathode and the materials which might evaporate from the surface of the cathode and condense on the beads. It is important in diodes of this type where exponential current-voltage characteristics are the desiderata that leakage currents between the electrodes be kept at a minimum. The auxiliary anode sections are electrically tied together by strap 16, the assembly being connected to lead-in base pin 17.

The lead-in conductor 18 for the anode 9 may be sealed in the press or in the side of the envelope as shown. Consistent with the low internal leakage requirements, the lead-in 18 is preferably sealed with the glass bead 19 in the metal guard ring 20.

In operation, the cathode is operated at any good electron-emitting temperature as in the 200K range, and the anode 9 is varied from sufficiently negative voltages to near zero or cathode potential, at which space charge current limitation begins. By keeping the potential of auxiliary anodes 10 and 11 equal to the anode 9 potential, the electric lines of force between the anode and the cathode are uniformly radial throughout the length of the anode including space opposite the upper and lower boundaries thereof. All boundary effects are eliminated by the diode of this invention.

Since it is desired to keep the potential of the anode section at the same level throughout the operating range of the diode, a feedback or servo circuit should be connected between the anode 9 and the anodes It and 11. Anode 9 could, for example, lead into the high impedance input of a vacuum tube ampliler, the output of which is so loaded and coupled as to cause the anodes 1011 to follow voltage variations of 9 without loading the circuit of anode 9.

It will be apparent that the round concentric electrodes disclosed could within the scope of this invention be developed into flat parallel planar electrodes with the anode 9 framed in and in the plane of the auxiliary anodes 1t11, and with the coated area of the cathode extending beyond the boundary of the anode 9.

While the principles of the invention have been described in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

1. A therminoic diode comprising: an enclosing envelope; a cathode within said envelope and having an extended electron-emitting surface; a main anode within said envelope having an extended surface uniformly spaced from said cathode emitting surface, said cathode emitting surface extending beyond the extremities of said main anode surface respectively; a pair of auxiliary anodes within said envelope arranged on either side of said main anode and insulatingly spaced therefrom, each of said auxiliary anodes having a portion adjacent said main anode overlying said cathode emitting surface and having the same spacing therefrom as said main anode and a portion remote from said main anode extending beyond said cathode emitting surface.

2 The combination of claim 1 in which said main anode is mechanically and insulatingly supported from said auxiliary anodes and said auxiliary anodes are directly electrically connected within said envelope.

3. In combination: a thermionic diode comprising an enclosing envelope, a cathode within said envelope and having an extended electron-emitting surface, a main anode within said envelope having an extended surface uniformly spaced from said cathode emitting surface, said cathode emitting surface extending beyond the extremities of said main anode surface respectively, and a pair of auxiliary anodes within said envelope arranged on either side of said main anode and insulatingly spaced therefrom, each of said auxiliary anodes having a portion adjacent said main anode overlying said cathode emitting surface and having the same spacing therefrom as said main anode and a portion remote from said main anode extending beyond said cathode emitting surface; and a feedback circuit connected between said main anode and said auxiliary anodes and arranged to maintain the potential of said auxilary anodes the same as that of said main anode.

4. A thermionic diode comprising: an enclosing envelope, a pair of spaced apart insulating spacers extending across said envelope; a cylindrical cathode in said envelope extending between and supported by said spacers and having an extended electron emitting surface terminating short of said spacers respectively; three tubular anodes in said envelope between said spacers disposed concentrically about said cathode in insulatingly spaced apart end-to-end relationship, the middle one of said anodes being of lesser extent than said cathode emitting surface, the outer ones of said three anodes extending respectively beyond said'cathode emitting surface, the outer ones of said three anodes being supported respectively from said spacers and being electrically directly connected within said envelope; and mechanical supports insulatingly supporting said middle anode from said outer anodes.

References Cited in the file of this patent UNITED STATES PATENTS 1,638,499 Mavrogenis Aug. 9, 1927 1,872,274 Goldsborough Aug. 16, 1932 1,917,739 Schroter July 11, 1933 2,263,660 Vasselli Nov. 25, 1941 2,309,220 Skellett Jan. 26, 1943 FOREIGN PATENTS 282,712 Great Britain Mar. 15, 1928 

